1. Field of the Invention
The present invention relates to a photoresin printing plate for use in printing a corrugated board. More particularly, the present invention is concerned with a photoresin printing plate for use in printing a corrugated board, which has a Shore A hardness at 20.degree. C. of 25 to 60, an impact resilience at 20.degree. C. of 35 % or more as measured by a falling ball method and a surface tack at 20.degree. C. of 40 g/cm or less and which is capable of providing prints having high precision of image with a high operation efficiency, that is, without necessity of interrupting the printing operation in order to clean the printing plate.
2. Description of the Prior Art
Rubber printing plates for use in printing a corrugated board have previously been used. However, it has not been possible to print a fine image with any degree of high precision using a rubber printing plate. In the recent years, it has become necessary for the image printed on a corrugated board to be finer. Accordingly, photoresin printing plates which are capable of providing a print having a fine image with high precision have been taking the place of rubber printing plates. Especially in the type of printing which requires a high degree of precision, such as bar code printing, the use of a photoresin printing plate is indispensable because a rubber printing plate simply cannot satisfy the requirement of high precision.
In printing a corrugated board, the following points must be taken into consideration. First, a corrugated board on which the image is to be printed does not always have a flat surface. Second, in general, in the field of printing, the uniform transfer of the ink can be achieved under a high printing pressure. However, in printing a corrugated board, the ink must be uniformly transferred onto the surface of the corrugated board under a low printing pressure because too high a printing pressure causes the flute of a corrugated material provided between the liners of the corrugated board to be deformed. This leads to a lowering of the strength of the corrugated board as a whole. In view of the above-mentioned points, a soft printing plate having a Shore A hardness of 25 to 60 is generally used for printing a corrugated board.
However, the use of a photoresin printing plate having such a low degree of hardness for printing a corrugated board involves a serious practical problem. In a printing operation, paper dust or the like present on the surface of the corrugated board to be printed sometimes adhere to the surface of the printing plate. In the case of a soft photoresin printing plate as mentioned above, once such an adhesion occurs, the paper dusts or the like cannot easily be released from the surface of the printing plate, making it necessary to interrupt the operation of the printing machine in order to clean the printing plate. This inevitably lowers the productivity and the yield of the prints.
For example, U.S. Pat. No. 3,960,572 discloses an unsaturated polyurethane type photosensitive resin composition for use in preparing a printing plate. In preparing the composition, a polyether diol and a polyester diol are used in an amount ratio of 1:4 to 3:1. This reference, however, does not disclose the capability of the printing plate to release the paper dust or the like which has adhered to the surface of the plate. The printing plate disclosed in this reference has a low impact resilience as measured by a falling ball method and, therefore, has a poor capability for releasing such types of dust or the like, as will be later discussed. In this connection, it is to be noted that although the reference discloses values of the impact resilience exceeding 35%, the indicated values of the impact resilience are those which are obtained by the measurement in accordance with the Dunlop method as substantiated by the reproduction of the examples in the reference. The value of the impact resilience of a printing plate as measured by the Dunlop method is usually about twice as high as the value of the impact resilience of the same printing plate as measured by a falling ball method. Therefore, in terms of the impact resilience as measured by a falling ball method, as is the case with the present invention, the reference does not disclose a value of 35% or more.
Further, U.S. Pat. No. 3,782,961 discloses a photosensitive resin composition for use in producing a flexographic printing plate which is suitable for printing packaging materials, including a corrugated board. The composition comprises a specific polyurethane as the prepolymer. However, this reference provides no disclosure as to the capability of the printing plate prepared from the composition, to release paper dust or the like, which has adhered to the surface of the plate.
Various proposals have heretofore been made for solving the above-mentioned problem. However, such proposals deal solely with a method for removing the surface tack of a photoresin printing plate, based on the assumption that the capability of a printing plate to release the paper dust or the like, which has adhered to the surface of the plate, depends mainly on the surface tack of the printing plate, as well as a method for improving the ease of handling of the printing plate (see, for example, U.S. Pat. No. 4,202,696).
By removing the surface tack of the photoresin printing plate in accordance with the proposed, methods, the capability of a photoresin printing plate to release the paper dust or the like, which has adhered to the surface of the plate is somewhat improved. However, these methods still have a problem that once paper dust or the like has adhered to the surface of a printing plate, more than several tens of prints are necessarily produced before the paper dust or the like is self-released from the printing plate and that, therefore, it is still necessary to interrupt the printing operation to clean the printing plate.